1. Technical Field
The invention relates to a method of controlling the nominal fill and purge times used in connection with an emission control device to facilitate xe2x80x9clean-burnxe2x80x9d operation of an internal combustion engine.
The invention relates to a method of optimizing the release of constituent exhaust gas that has been stored in a vehicle emission control device during xe2x80x9clean-burnxe2x80x9d vehicle operation.
2. Background Art
Generally, the operation of a vehicle""s internal combustion engine produces engine exhaust that includes a variety of constituent gases, including carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). The rates at which the engine generates these constituent gases are dependent upon a variety of factors, such as engine operating speed and load, engine temperature, spark timing, and EGR. Moreover, such engines often generate increased levels of one or more constituent gases, such as NOx, when the engine is operated in a lean-burn cycle, i.e., when engine operation includes engine operating conditions characterized by a ratio of intake air to injected fuel that is greater than the stoichiometric air-fuel ratio, for example, to achieve greater vehicle fuel economy.
In order to control these vehicle tailpipe emissions, the prior art teaches vehicle exhaust treatment systems that employ one or more three-way catalysts, also referred to as emission control devices, in an exhaust passage to store and release select constituent gases, such as NOx, depending upon engine operating conditions. For example, U.S. Pat. No. 5,437,153 teaches an emission control device which stores exhaust gas NOx when the exhaust gas is lean, and releases previously-stored NOx when the exhaust gas is either stoichiometric or xe2x80x9crichxe2x80x9d of stoichiometric, i.e., when the ratio of intake air to injected fuel is at or below the stoichiometric air-fuel ratio. Such systems often employ open-loop control of device storage and release times (also respectively known as device xe2x80x9cfillxe2x80x9d and xe2x80x9cpurgexe2x80x9d times) so as to maximize the benefits of increased fuel efficiency obtained through lean engine operation without concomitantly increasing tailpipe emissions as the device becomes xe2x80x9cfilled.xe2x80x9d The timing of each purge event must be controlled so that the device does not otherwise exceed its NOx storage capacity, because NOx would then pass through the device and effect an increase in tailpipe NOx emissions. The frequency of the purge is preferably controlled to avoid the purging of only partially filled devices, due to the fuel penalty associated with the purge event""s enriched air-fuel mixture.
Thus, for example, U.S. Pat. No. 5,437,153 teaches an open-loop method for determining appropriate device fill times wherein an accumulated estimate of instantaneous engine-generated NOx (all of which is presumed to be stored in the device when operating in a linear operating range) is compared to a reference value representative of the instantaneous maximum NOx-storing capacity of the device, determined as a function of instantaneous device temperature. When the accumulated estimate exceeds the reference value, the xe2x80x9cfillxe2x80x9d is deemed to be complete, and lean engine operation is immediately discontinued in favor of an open-loop purge whose duration is similarly based on the estimated amount of stored NOx.
The prior art has recognized that the storage capacity of a given emission control device is itself a function of many variables, including device temperature, device history, sulfation level, and the presence of any thermal damage to the device. Moreover, as the device approaches its maximum capacity, the prior art teaches that the incremental rate at which the device continues to store the selected constituent gas may begin to fall.
Accordingly, U.S. Pat. No. 5,437,153 teaches use of a nominal NOx-storage capacity for its disclosed device which is significantly less than the actual NOx-storage capacity of the device, to thereby provide the device with a perfect instantaneous NOx-storing efficiency, that is, so that the device is able to store all engine-generated NOx as long as the cumulative stored NOx remains below this nominal capacity. A purge event is scheduled to rejuvenate the device whenever accumulated estimates of engine-generated NOx reach the device""s nominal capacity.
The amount of the selected constituent gas that is actually stored in a given emission control device during vehicle operation depends on the concentration of the selected constituent gas in the engine feedgas, the exhaust flow rate, the ambient humidity, the device temperature, and other variables. Thus, both the device capacity and the actual quantity of the selected constituent gas stored in the device are complex functions of many variables.
It is an object of the invention to provide a method and system by which to optimize the fill time during which a constituent gas of the engine-generated exhaust gas is stored in a vehicle emission control device.
Under the invention, a method is provided for optimizing the fill time of an emission control device located in the exhaust passage of an engine upstream from an oxygen sensor, wherein the emission control device is filled with a constituent gas of engine-generated exhaust gas during a first engine operating condition and being purged of previously-stored constituent gas during a second engine operating condition. The method includes optimizing the purge time for a given fill time to provide a purge time adjustment multiplier related to device capacity; and adjusting the given fill time based on a function of the multiplier to achieve storage of enough of the constituent gas to fill the device to a predetermined fraction of the device capacity. More specifically, in a preferred method of practicing the invention the step of optimizing the purge time includes producing a purge time correction factor based on the error between a desired saturation time and a calculated saturation time, the calculated saturation time based on a characteristic of the output of the sensor following the given fill time; storing the magnitude of a final purge time correction factor for the given fill time; increasing the fill time by a predetermined amount and performing purge optimization operations for the new fill time; storing the magnitude of the final purge time correction factor for the new fill time; determining the absolute difference between the final purge time correction factors for the given and new fill time; and, if the difference is less than a predetermined value, decreasing the fill time by the predetermined amount, and otherwise increasing the fill time by the predetermined amount and repeating the process until an optimum fill time and an optimum purge time are achieved.
In accordance with another feature of the invention, in a preferred method of practicing the invention the step of adjusting the fill time includes iteratively determining an adjusted fill time by adjusting the initial fill time by a plurality of predetermined increments, optimizing an adjusted purge time corresponding to the adjusted fill time, calculating a difference between the adjusted purge time and the initial purge time, and comparing the difference with a predetermined target value, until the difference is less than a predetermined target value.